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Caffeine (C₈H₁₀N₄O₂) is a chemical stimulant of the central nervous system. It belongs to a class of organic compounds called alkaloids, which also include morphine, codeine, LSD, cocaine and nicotine. Caffeine is found in a variety of products, including certain supplements, headache medications, coffee, tea, most sodas, sports drinks and chocolate. Varying amounts of it can be found in more than 60 plants, including the following:

• Cocoa beans
• Coffee beans
• Cola (kola) nuts
• Guarana
• Yerba mate leaves
• Tea leaves

People have been consuming caffeine since the first pot of tea was brewed in China around 2700 B.C. It is widely used for its ability to help people wake up, stay alert, maintain energy levels and respond quickly. Some estimates suggest that almost 50 percent of people in the United States have at least one cup of coffee per day, and about 70 percent of people drink tea in the United Kingdom. Some researchers claim that caffeine is nonaddictive, whereas other researchers declare it to be the fifth most addictive substance in the United States (following heroin, alcohol, cocaine and nicotine). Caffeine also is added as a flavoring agent to some foods and beverages.

Once consumed, caffeine quickly enters the bloodstream through the stomach and small intestine. It travels through the bloodstream and goes directly to the central nervous system. There, it stimulates certain chemicals in the brain that produce an energizing effect on the body. This effect can be felt approximately 15 to 45 minutes after consumption, and reaches a peak within 30 to 60 minutes. Caffeine does not accumulate in the bloodstream nor is it stored in the body.

The half-life of caffeine is roughly five to seven hours, meaning that it will take about that long for 50 milligrams (mg) of caffeine to be metabolized (broken down) after 100 mg have been consumed (e.g., a cup of coffee contains 60 to 160 mg of caffeine). Until the liver has completely broken down the caffeine, the central nervous system will continue to be stimulated. Once broken down, caffeine is quickly expelled from the body through urine, breast milk, saliva or semen.

Research has revealed that caffeine’s stimulating effect is due to its relationship with a chemical called adenosine, which appears to be the body’s natural sleep-enhancer. Normally, an adenosine molecule fits neatly into the receptor on a cell (like a key fitting in a lock), which turns on the cell’s activity and facilitates sleep. However, caffeine fills some of those receptors so that fewer adenosine molecules can get in to turn on the cells’ activity. Consequently, the body’s natural sleep-enhancing effect is partially blocked, and the person feels less fatigued and more alert.

Caffeine also has several metabolic effects, including:

• Increases movement of food through the intestinal tract
• Releases free fatty acids from adipose (fat) tissue
• Affects kidneys and increases urination